The present invention relates generally to communication systems and, in particular, to a method and apparatus for performing baseband data slicing.
Communications systems are well known in the art. In many such systems, information (e.g., voice or data information) to be conveyed from a transmitting communication unit to a receiving communication unit is represented as a baseband signal that, in turn, is used to modulate a carrier signal. At the receiving communication unit, a demodulation process is employed to extract the baseband signal from the carrier signal. Where the baseband signal is a representation of digital data, so-called data slicing is performed at the receiver in order to determine what binary digits have been received. In general, data slicing refers to the process whereby the recovered baseband signal is compared against a threshold to decide whether, for a given data period, a binary one or zero value has been sent. Various techniques for performing such baseband data slicing are known in the art.
FIG. 2 illustrates a prior art apparatus for performing AC-coupled baseband data slicing. In the present context, the term AC-coupled refers to the fact that only higher frequency components of the baseband signal are compared against the threshold. As shown in FIG. 2, a demodulator 202 using known demodulation techniques outputs a recovered baseband signal. The baseband signal output by the demodulator 202 includes, as is often the case, low frequency components including a DC component or offset. An exemplary baseband signal Vin is illustrated in FIG. 3 as a voltage varying over time. For the sake of simplicity, the baseband signal 302 shown in FIG. 3 is represented as a single sinusoidal waveform. In practice, however, such baseband signals typically comprise a plurality of frequency components, resulting in a more complex time domain waveform. As shown, the baseband signal comprises an offset component VDC 304. In an ideal system, the baseband signal is assumed to adhere to a 50% duty cycle. That is, a binary one value (represented, for example, by a positive voltage) has a duration in the time domain baseband signal equal to a binary zero value (represented, for example, by a negative voltage). In order to accurately compare the received baseband signal against a threshold, and still assuming the 50% duty cycle constraint, the AC-coupled data slicer filters out all low frequency components, including the offset VDC using a high pass filter 204. In effect, the high pass filter causes the baseband signal to become centered upon a zero voltage value, rather than a DC offset value. This is illustrated in FIG. 4, where the high pass filtered version of baseband signal 402 is shown over time.
As known in the art, such high pass filters are characterized by a settling time constant which causes DC components in the input signal to be removed from the filter output according to a decaying exponential curve. This effect is illustrated in FIG. 4 where the DC component 404 is gradually removed from the filtered baseband signal 402. Progressively greater attenuating effects by the high pass filter are typically characterized by a correspondingly increased settling time constant that implies, in turn, that the filter output will increasingly lag behind the filter input. As such, filter design must typically strike a balance between the desired attenuation and the settling time constant that the system is able to endure.
Referring again to FIG. 2, the high pass filtered baseband signal is provided to a comparator 206 that compares the filtered signal against a threshold Vth. Because the high pass filter 204 has the effect of centering the baseband signal on a zero voltage or ground level, Vth is preferably set to that level. Typically, the comparator will output a predetermined positive voltage for any input signal above the threshold and a predetermined negative voltage (of same magnitude) for any input below the threshold. For example, assuming that the filtered baseband signal 402 of FIG. 4 is applied to the comparator of FIG. 2, an exemplary output Vout 502 is shown as the heavy dashed line in FIG. 5. Note that the output 502 of the comparator 206 does not accurately track the baseband signal 302 output by the demodulator 202 due to the settling time constant of the high pass filter. In an ideal system, in which the settling time constant where negligible, the output of the comparator 206 would correspond to the curve having reference numeral 504 in FIG. 5. In a typical application, however, a significantly long predetermined delay 406 (typically several times the length of the settling time constant) must pass before the data output of the comparator may be considered reliable. As a result, data may be lost at the beginning of a received signal. To combat this problem, a sufficient amount of dummy data may be inserted as a preamble to the baseband signal being transmitted such that the processing of the dummy data at the receiver allows the predetermined delay to pass before actual baseband data is demodulated and sliced. However, this approach builds in a fixed signal delay that may not be acceptable in all applications. A further difficulty with this approach is that long periods of zeros or ones represented by the baseband signal are seen by the high pass filter as a DC component to be filtered out, thereby attenuating the desired signal, which results in decreased system performance.
An alternative to the AC-coupled method is the DC-coupled method of FIG. 6. In this method, an unfiltered version of the received baseband signal is provided to the comparator and a filtered version of the DC offset is provided as the threshold provided to the comparator. In a sense, instead of filtering the baseband signal to the correct threshold level as in the AC-coupled method, the DC-coupled method filters the baseband signal to determine the necessary threshold level. To this end, a low pass filter 604 is used to attenuate all higher frequency components from the baseband signal, preferably leaving only the DC offset to be used as the comparator threshold input. However, as with the high pass filter 204, the low pass filter 604 is characterized by a settling time constant, implying that there is a lag between the time that the DC offset is applied to the low pass filter and the time that it is reflected in the output signal. This is illustrated in FIG. 3 where the output of the low pass filter Vlpf 306 is shown as a dotted line. Because of the settling time constant, the output of the comparator in FIG. 6 will be similar to the filtered output 502 shown in FIG. 5. That is, the comparator output will not be reliable until after the predetermined delay has passed. Furthermore, the long strings of ones or zeros in the baseband signal will cause the output of low pass filter 604 to drift, causing a corresponding change in the threshold level. In essence, the DC-coupled approach illustrated in FIG. 6 suffers from the same disadvantages as the AC-coupled approach.
A variant of the DC-coupled approach is to set the threshold to an initial, predetermined value equal to the expected offset value in the demodulator output, rather than low pass filtering to determine the threshold. This approach suffers, however, in the event that perturbations resulting from the transmission channel or the receiver front-end cause the actual offset value to differ from the assumed offset value, thereby resulting in the use of a non-optimal threshold value. To combat the possible drift of the offset value, yet another approach is to continuously adjust the threshold level as illustrated in FIG. 7.
The threshold is initially set to a predetermined value Vth(0). As shown in FIG. 7, the output of the comparator is passed through a low pass filter 708 that has the effect of averaging the duty cycle of the comparator output. If the threshold is, for example, too low, the comparator output will have excessively long high periods, e.g., a duty cycle greater than 50%. The low pass filter will reflect this increase in duty cycle by an increasingly positive output value. This increased low pass filter output is then added to the initial threshold via an adder 710, thereby tending to correct the threshold value. The same process works in the opposite manner when the threshold is too high, resulting in excessively long low periods, e.g., a duty cycle less than 50%. In this case, the output of the low pass filter will become increasingly negative, thereby causing the actual threshold value to be decreased. However, as with the other filter-based approaches discussed above, this method is adversely affected by long strings of binary one and zero values. This is illustrated in FIG. 8 where an exemplary comparator output waveform 802 and the corresponding output 804 from the low pass filter 708 are shown. Note that long strings of binary zero values (shown as negative voltages in the output waveform 802) and binary one values (shown as positive voltages in the output waveform 802) cause the output 804 of the low pass filter 708 to drift. The magnitude of this drift will depend on the actual filter implementation used and the length of the strings of ones and zeros encountered. However, the overall effect of this drift is to cause unnecessary changes to the threshold, thereby degrading system performance.
Thus, a need exists for an improved data slicing technique that decrease the effects of filter settling times typically encountered in AC-coupled and DC-coupled data slicing techniques.
The present invention provides a technique for data slicing that substantially overcomes the performance degradations caused by filter settling times in prior art techniques. A DC-coupled data slicer and an AC-coupled data slicer are provided, both performing data slicing on a baseband signal based on a variable threshold and a fixed threshold, respectively. The variable threshold is initially set to a stored threshold value, which stored threshold value represents a previously used value of the variable threshold. Differences between DC-coupled sliced data and the AC-coupled sliced data are determined and used to adjust the variable threshold. In a preferred embodiment, a comparator in the form of an exclusive-OR gate is used to determine the differences between the DC-coupled and AC-coupled sliced data to provide a threshold error signal that is, in turn, converted to adjustments providing a threshold correction signal. The AC-coupled data slicer is characterized by a settling time constant. Thus, the variable threshold is not adjusted until expiration of a predetermined delay preferably set to be a multiple of the settling time constant. During pendency of the predetermined delay, the DC-coupled data slicer provides reliable data. After expiry of the predetermined delay, adjustments to the variable threshold are made to correct any detected variances in the expected duty cycle of the DC-coupled data slicer output. In this manner, the present invention overcomes the problems resulting from settling times inherent in prior art techniques.